1. Field of the Invention
This invention relates generally to telephone communication circuitry, and more particularly to circuitry designed for boosting an incoming ringing signal to ring several telephones on the same line.
2. Brief Description of the Prior Art
Telephone signals are sent from the telephone company's central switching office over the central switching office lines to a subscriber's telephone. The subscriber may have one or more telephones connected to a single line. Each telephone introduces additional resistance to the subscriber line. As a telephone ringing signal is sent from the telephone company's central switching office over the central switching office lines to the subscriber lines, its amplitude is diminished as it passes through the ringing apparatus of each telephone on the subscriber line. As more telephones are added, a point is reached where the voltage from the central switching office is no longer powerful enough to trigger the ringing mechanism of the telephones. When this happens, the telephones fail to ring even though a ring signal is being sent from the central switching office. The subscriber is therefore not aware that he is being called.
Several devices have been made to resolve this problem. There are those devices which process the incoming signal and send this processed signal to the subscriber line, and those devices which generate their own ringing signal locally and send this signal to the subscriber line.
U.S. Pat. Nos. 3,746,795, 3,763,320, 3,781,480, and 4,500,844 disclose devices which process the incoming signal from the central switching office. Each employs a different type of modulation and amplification to boost the signal received. This boosted signal is output to the subscriber line. The object of each of those devices is to amplify the signal such that the ring signal is great enough to drive numerous telephones on the subscriber line.
In the process of amplifying the signal sent from the central switching office, noise inherent in the signal is also amplified. The modulation schemes employed by each device are aimed at minimizing the noise in the signal passed onto the subscriber line.
The second group of devices, such as disclosed in U.S. Pat. Nos. 4,276,448 ("Embree"), No. 4,723,275 ("Hirth"), and No. 4,827,503 ("Takado") create a locally generated, high amplitude, ring signal which is passed on to the subscriber line.
Hirth shows a method of switching between speech and ring circuitry, a sensing means to sense when the ring signal from the central switching office is being received, and a local ring signal generating means. The Hirth device has a sensing means to determine if the local ring signal generator is functioning, but does not pass the ring signal from the central switching office on to the subscriber line in the event of a ring signal generator failure. The Hirth device is driven by a microprocessor.
Embree discloses a device which senses a ring signal and generates its own local ring signal. This ring signal can then be applied to the subscriber line. Embree has the capability of driving multiple ringers on the same subscriber line and producing multiple tones as a ring signal. The device also has circuitry to differentiate between dialing pulses and voice signals, as well as circuitry to make the device insensitive to line voltage variations. Embree discloses a device which can only drive an electro-acoustic transducer.
Takado discloses a ring signal generator which can be switched onto the subscriber line. Takado does not have a sensing circuit to determine if the central switching office is sending a ring signal. Takado discloses essentially a ring signal generator which is not responsive to an incoming ring signal from the central switching office, and must be manually switched onto the subscriber line.
U.S. Pat. No. 4,174,467 ("Ferrieu") discloses a device that senses the incoming ring signal from the central switching office, and uses this signal to synchronize a local ring generator. This local ring generator is not switched on-line, but inductively creates a ring signal on the subscriber line. Ferrieu monitors the incoming ring signal and creates a control signal which drives switches in the local ring signal generator at roughly twice the frequency of the incoming signal, to synthesize size a ring signal. This control signal is produced by means of a delta modulator sampling the incoming ring signal.
Since the central switching office line usually travels a great distance, it is prone to high power interference, such as a lightening strike. This central switching office line eventually ends at one of the telephones in the subscriber lines, introducing a possibility of passing the power through the telephone lines and injuring persons or equipment on the subscriber line. Electronic isolation means are provided in some devices to protect persons from injury and damage to equipment. U.S. Pat. No. 4,741,031 ("Grandstaff") discloses one such device. These isolation devices are designed to transmit up to a maximum power defined by the design of the device. Any power above the maximum is simply dissipated through a ground.
There is a need for a simple and inexpensive device which is capable of boosting the ring signal from the telephone company to a level which is high enough to ring several telephones on the same line. There is also a need for a device which boosts the ring signal on the subscriber line which can use a variety of differing types of local generators. This would allow the device to be adapted to ring differing types of telephones.